New study showing Marin among hot spots for breast cancer in California provides no new clues to the cause

A new report on breast cancer incidence in California provides valuable new data but no new clues as to why rates are higher in Marin and other areas of the state, local health officials said Tuesday.

The study released Monday by the Public Health Institute's California Breast Cancer Mapping Project in Oakland identified four areas of California where invasive breast cancer rates were 10 percent to 20 percent higher than the state average between 2000 and 2008.

The "areas of concern" included parts of the north and south Bay Area, including Marin, and sections of Ventura, Los Angeles, Riverside and Orange counties. Unlike previous reports on breast cancer incident rates, this one used United States Census data to map rates across county boundaries.

"The more detailed census tract analysis is good," said Larry Meredith, director of the Marin County Department of Health and Human Services. "It indicates there is a greater population at risk, so I think it will encourage additional studies."

That breast cancer rates are higher than average among women living in Marin has long been known. Concern about Marin's high incidence of breast cancer reached a fever pitch in 2002 when the Northern California Cancer Center reported that the rate of breast cancer in Marin averaged 198.5 cases per 100,000 women per year for white women during 1995-99 — six times greater than any other California county.

That number, however, was revised downward significantly after further analysis.

The average annual incidence rate of female breast cancer among non-Hispanic whites in Marin County from 2007 to 2009 — the most current data available — was 153 per 100,000, compared to a rate of 142 per 100,000 statewide.

In the past, health officials have attributed Marin's higher incidence rates to several risk factors that tend to be common among affluent, well-educated women: alcohol consumption, delay of childbearing until after age 29, and the use of hormone therapy following menopause. After news of the link between hormone therapy and breast cancer broke in 2002, breast cancer rates in Marin immediately began to fall.

Dr. Eric Roberts, a research scientist at Public Health Institute and principal investigator of the California Breast Cancer Mapping Project, said, "I would say these findings are consistent with that idea."

Christina Clarke, a research scientist with the Northern California Cancer Center in Union City, said, "We do know there is a very particular socio-demographic profile of areas that tend to have high breast cancer rates."

Clarke said for the most part the areas of concern identified in the new study are places where predominately white, well-educated women live.

"I don't think the report gives us any new hints about why these areas have higher rates of breast cancer," Clarke said. "But it does help us understand which communities in California are bearing the highest burden of the disease."

She said that information could prove useful when government officials are deciding how to allocate health care resources.

Janice Barlow, executive director of the San Rafael nonprofit Zero Breast Cancer, cautions, however, against relying too heavily on demographics to explain the higher incident rates. She notes that not all of the areas of concern are enclaves of affluence.

"You can't convince me that the demographics in Richmond are similar to the demographics in Marin County or even Solano County," Barlow said. "It can't be explained by just the demographics."

Rochelle Ereman, an epidemiologist with the county of Marin, said, "All this study does is tell you about incident rates. It really doesn't tell you anything about risk factors and whether these areas have similar risk profiles. That would be a very good next step."

Both Clarke and Barlow also noted that the study fails to differentiate between the types of breast cancer the women in these areas of concern are contracting.

Clarke said, "This report treats breast cancer as one entity. All the cool, meaningful stuff that we're doing now splits breast cancer into molecularly different subtypes."

This is important, Barlow said, because some breast subtypes have different risk factors. For example, Barlow said for a subtype of breast cancer, known as triple-negative, "having more children at a younger age is now considered a risk factor."